Ion sources have a number of uses in industry and research. For example, they are used in secondary ion mass spectrometers, ion microprobes, heavy ion probes, fast atom bombardment mass spectroscopes, and in microelectronic circuit fabrication and for space propulsion.
One type of ion source is the contact or surface ionization ion source. Contact ionization ion sources typically produce beams of cesium ions. A conventional contact ionization ion source for cesium ions is shown in G. R. Brewer, Ion Propulsion: Technology and Applications, (Gordon and Breach, 1970), pp. 102-105. The contact ionization ion source includes a contact ionizer composed of grains of a refractory metal such as tungsten which are pressed and sintered into a porous matrix. The ion source also includes an oven for vaporizing cesium metal and a heated manifold which connects the oven to the porous matrix of refractory material. Cesium is vaporized in the oven at a temperature of about 300.degree. C. and conducted to one side of the contact ionizer by way of the heated manifold. The cesium atoms flow into the voids between the grains of tungsten of the ionizer and, depending on the temperature and other factors, interact with the tungsten and become ionized. The cesium evaporates from the ionizer as ions. A disadvantage of such contact ionization ion sources is the requirement of an oven for containing and heating a supply of cesium to vaporize the cesium and a heated manifold to conduct the cesium vapor to the ionizer.
When solid alkali and alkali-earth electrolytes are heated to a temperature of about 900.degree. C. or greater, they typically emit positive ions. This thermionic emission phenomenon has been exploited to construct solid state ion sources.
An example of a thermionic emission solid state ion source is described in O. Heinz and R. T. Reaves, "Lithium Ion Emitter for Low Energy Beam Experiments," Rev. Sci. Instr., vol. 39, pp. 1229-1230 (August 1968). In the Heinz and Reaves article, a lithium ion emitter is disclosed for low energy ion beam experiments. The emitter includes a highly porous tungsten plug mounted on a molybdenum body. A cavity in the molybdenum body contains a heater coil potted in high purity Al.sub.2 O.sub.3 for heating the porous tungsten plug indirectly. One of the two lithium-ion-emitting compounds, beta-eucryptite (Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2) or spodumene (Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2), is melted into the porous tungsten plug. When the plug containing beta-eucryptite or spodumene is heated to a temperature in excess of about 900.degree. C., positive lithium ions are emitted from the surface of the plug. In FIG. 2 of the article a circuit incorporating the lithium-ion emitter is shown. The emitter is located in front of and spaced apart from a grid electrode, which in turn is located in front of and spaced apart from a target electrode. The entire emitter assembly is biased above ground. The grid electrode is biased at below ground and the target electrode is grounded.
Another example of a thermionic emission solid state ion source is described in D. W. Hughes, R. K. Fenney, and D. N. Hill, "Aluminosilicate-Composite Type Ion Source of Alkali Ions," Rev. Sci. Instr., vol. 51, pp. 1471-1472 (November 1980). The source includes a layered pellet prepared by sintering a layered mixture of varying amounts of molybdemum metal powder and a powdered aluminosilicate containing an oxide of the desired alkali element. The concentration of the aluminosilicate increases relative to the metal layer-by-layer from zero percent aluminosilicate in a base layer to fifty percent by weight molybdenum-fifty percent by weight aluminosilicate in a top layer. The pellet is bonded to a modified cathode heater assembly by brazing the pure molybdenum base layer of the pellet to a surface of the heater assembly. The resulting ion emitter assembly is mounted behind a set of electrostatic focusing and accelerating electrodes.
A disadvantage of conventional thermionic solid state ion sources is that the energy of the ions emitted from such conventional ion sources is not well defined due to a voltage drop across the ion-emitting material. Furthermore, the thickness of the ion-emitting material must be kept small to ensure that the voltage drop across the ion-emitting material is acceptable. However, having a small thickness of the ion-emitting material limits the average useful lifetime of such sources since the number of ions that can be stored in the ion-emitting material is limited.